(1) Field of the Invention
This invention relates to a signal processing apparatus and a method of signal processing. More particularly, but not exclusively, the invention relates to a signal processing apparatus and method of signal processing for use in multiple input multiple output (MIMO) antenna architecture.
(2) Description of the Art
Current iterative processing techniques allow wireless telecommunications data transmission rates for links between single antenna transceivers that approach the Shannon data capacity limit, for a single input single output (SISO) system. The Shannon data capacity limit is given by:C=W log2(1+S/N) bit/sWhere:C is the channel capacityW is the channel bandwidth in HzS/N is the signal-to-noise power ratio.
The concept of spatial multiplexing allows the realisation of enormous data transmission rates when multiple transmit and receive antennas are employed. For a given average signal-to-noise ratio (SNR) per receive antenna, or total transmit power, the theoretical MIMO capacity increases linearly with the number of transmit or receive antennas, whichever is the smaller, at high SNR values. Therefore, it is desirable to implement systems with large MIMO configurations, for example four or more transmit antennas (Tx) and four or more receive antennas (Rx), or more. These large MIMO configurations are typically integrated with iterative processing in order to provide reliable transmission of data at high spectral efficiencies, an example of such a prior art MIMO architecture combined with iterative decoding is shown in FIG. 1.
One problem associated with large MIMO architectures is a high computational burden that is imposed upon them by the execution of Log Likelihood Ratio (LLR) calculations at a front end of an iterative decoder on the receive side of the MIMO architecture. This is due to the LLR calculations having to account for every possible candidate symbol across every transmit antenna. These calculations require marginal probabilities to be obtained by summing over all symbol probabilities to obtain bit LLR values. The number of these calculations grows exponentially with the number of transmit antennas and the modulation level. The number of calculations that must be executed to obtain LLR value could be thousands, tens of thousands or even up to millions per second.